Clarification relates to reducing solids content water or other liquid streams, for example syrups and oils, which cannot be efficiently removed solely by mechanical filtration methods. Often a clarification process is used to remove non-dissolved solids before further waste processing, or may be applied to provide water which is clean enough to recycle into the same process even if not clean enough for discharge. Dissolved aeration flotation (DAF) is a widely used method to remove organic contaminants from wastewater streams such as from food processing plants. The basic process consists of injecting water saturated with gas—either air or another gas selected to be less reactive with a particular waste chemistry—into a flotation tank (“aeration”) where the gas comes out of solution forming bubbles which float to the surface of the tank. The aerated water is created by dissolving gas into the water in a high pressure environment until it reaches saturation level at that high pressure. When the gas-saturated water at high pressure is depressurized the gas comes out of solution. Bubble size and density can be controlled by varying, among other things, the maximum saturation pressure and the rate of depressurization. The rising gas bubbles adhere to particulates in the wastewater and lift them to the surface where they are skimmed off. The floating particulate matter is referred to as “retentate”, and after removal is referred to as “sludge”. Aeration may be accomplished using pressurized saturation tanks or pumps designed for the purpose, such as aeration turbine pumps.
Flocculate agents may be mixed with the wastewater prior to aeration to react with or bind to particulates, creating larger and less dense suspended coagulated particles which are more susceptible to binding with gas bubbles and thereby more effectively driven to the surface for removal. Many flocculating chemicals are known and selected based on the anticipated chemistry of the waste stream and the expected downstream uses of the clarified effluent and retentate sludge. Retaining wastewater in a flotation tank exposed to aerated water for a longer period provides greater removal effectiveness.
Although described in conjunction with a DAF wastewater treatment system, the effluent weir apparatus is useful for other clarification systems and methods, including for example simple settling tank systems without added chemicals or separation plates, or heated settling tanks such as used for lube oil systems. In this application, “settling tank” and “separation tank” are used in their general sense, and intended to be interchangeable.
Additional problems arise with conventional effluent weir designs, which generally comprise either an edge trough at the liquid operating level of the tank, or a submerged weir pipe with perforations distributed along its surface, including the top surface. These designs allow particulates to enter the trough weir pipe and foul the pipe, reducing flow and potentially contaminating the effluent discharge. This design also creates a problem of sediments accumulating at the overflow edge of the edge trough, or on the relatively flat upper surface of conventional submerged weir pipes, which periodically dislodge and create spikes of particulates in the effluent, especially in weir pipes with top inlets. Maintenance requirements are substantially increased due to more frequent flushing required and more difficult cleaning during shutdowns.
Mounting the effluent weir apparatus transverse to the bulk flow within a separation tank aids in further slowing fluid velocity, improving settling out of heavier particulates, without interfering with laminar flow in the bulk fluid above the effluent weir.
Thus, there is a need for an improved effluent weir apparatus for fluid treatment systems that: (1) is compact; (2) can be retrofitted to existing separation tanks/systems to improve efficiency of legacy systems; (3) provides improved methods for removing effluent; (4) reduces buildup of sediments on surfaces; (5) provides improved solids removal efficiency; (6) improves laminar flow within the separation vessel; (7) reduces water velocity within the separation vessel; (8) improves dwell time within the separation vessel; (9) provides for adjustable height risers to control system liquid level; and, (10) improves overall efficiency and cost effectiveness.